FMR1 and fragile X syndrome: While FXS is mostly caused by the loss-of-function of wild-type FMRP, which results in neurodevelopmental and behavioral disorders characterized by mental retardation, autism, anxiety and mood instability, FXTAS is mostly caused by the CGG repeat expansion that produces gain-of-function RNA toxicity in the CNS, which results in a late-onset neurodegenerative disorder, characterized by progressive intention tremor, gait ataxia, parkinsonism, and cognitive decline (Jacquemont et al., 2007; Tassone and Hagerman, 2012).